The threshold for action potential generation and the frequency of firing of central neurons depend critically on the cell surface density, localization, and functional properties of Na channels. Control of the cell surface density and localization of Na channels is a critical aspect of neuronal function. Previous results suggest that assembly, cell surface insertion, and differential targeting of Na channel subtypes all play important roles in this process. Cloning and functional analysis of the pi and P2 subunits of brain Na channels have implicated these two proteins in modulation of Na channel gating, assembly of functional channels, and expression and localization of Na channels on the cell surface. In addition, the immunoglobulin- like folds in the extracellular domains of the p subunits suggest that they function as cell adhesion molecules. In our recent researc.1, we have defined the new structural requirements for modulation of gating by the pi subunit,discovered novel protein -protein interactions of P subunitswith the extracellular matrix protein tenascin, the cell adhesion molecule neurofascin, and the transmembrane receptor phosphoprotein tyrosine phosphatase P, and analyzed the functional roles of P subunits in vivo in mice with disrupted P subunit genes. In the next project period, we plan to identify the molecular deter- minant:* for functional interaction of Na channel a and P subunits,investigate the molecular determinants for interaction of Na channel P subunits with cell adhesion and extracellular matrix molecules, define the molecular basis for regulationof Na channels by associated protein tyrosine kinases and phosphoprotein tyrosine phosphatases., identify novel molecular compo- nents of Na channel signaling complexes and determine their functional role, and examine the functional role of Nachannel P subunitsand signaling complexes in vivo in genetically altered mice. Our results will add substantiallytounderstanding of the cell biology of the Na channel and the functional significance of Na channel signaling complexes.